(535h) The Effect of a Yield Stress on the Drainage of the Thin Film Between Two Colliding Newtonian Drops

A long-standing problem in oil sands extraction is the presence of 2-3% of residual water in the form of very fine drops (about 1 micron in size) in the diluted bitumen product. A strategy that has been evaluated many times in the past to remove this water is to coalesce the small water droplets with themselves or with bigger water droplets to produce larger drops that can be separated by conventional methods such as inclined settling and centrifugation. Unfortunately, these studies have indicated that these water-in-bitumen emulsions are extremely difficult to break¹. In the literature, the prevalent explanation for this stability is that molecules in bitumen called asphaltenes adsorb at the oil-water interface and render it rigid, thus strongly increasing the time required for coalescence. Recently, an alternative explanation has emerged to explain this stability², which suggests that bitumen is a fluid with a low yield stress due to the capability of the asphaltenes to associate with themselves. During the process of hydrodynamic drainage, if the stresses within the thin film fall below the yield stress of the asphaltene network, the film will stop draining, before non-hydrodynamic effects come into effect.

The objective of this paper is to assess the validity of the yield-stress explanation for water-in-bitumen emulsion stability. A survey of the literature reveals that a theoretical understanding of the drainage of the thin film of Bingham fluid between two colliding drops is still lacking, with the exception of two studies that make ad-hoc assumptions about the film shape^3,4. In this work, we examine this problem for low capillary numbers via a combination of scaling analysis and detailed numerical simulations based on the lubrication analysis^5,6. One obvious trend is that the introduction of a yield stress in the suspending fluid retards the drainage process relative to a Newtonian fluid of the same viscosity. But there are at least other three notable features of the film drainage process of Bingham fluids. First, the presence of yield stress prolongs the spherical regime of the drainage process and delays transition into the dimpled regime. Second, the drainage time shows a minimum with respect to the force applied to push the drops towards each other. Third, drainage may be arrested completely below a critical height due to the yield stress. This critical height scales as the square of the yield stress, the cube of the drop radius and inversely as the square of the interfacial tension. Counterintuitively, the critical height independent of the force colliding the two drops! This and other distinguishing characteristics of the drainage process will be elucidated in the presentation. We will conclude the presentation with an evaluation of the â??yield stressâ?? explanation for the stability of water-in-bitumen emulsions, outlining the regime of parameters for which this explanation may be valid.

References

1. F. Rao and Q. Liu, â??Froth Treatment in Athabasca Oil Sands Bitumen Recovery Process: A Reviewâ?, Energy Fuels 27, 7199â??7207 (2013).

2. P. Tchoukov, F. Yang, Z. Xu, T. Dabros, J. Czarnecki, and J. SjoÌ?blom, "Role of Asphaltenes in Stabilizing Thin Liquid Emulsion Filmsâ? , Langmuir 30, 3024â??3033 (2014).

3. S. Hartland and S. A. Jeelani, "Drainage in Thin Planar Non-Newtonian Fluid Films," Can. J. Chem. Eng. 65, 384-390 (1987).

4. S. Hartland and S. A. Jeelani, "Drainage of Thin Dimpled Non-Newtonian Fluid Films," J. Phys. Chem. 90, 6054-6059 (1986).

5. A. Saboni, C. Gourdon and A. K. Chesters, "Drainage and Rupture of partially mobile films during coalescence in liquid-liquid systems under a constant interaction force," J. Colloid Int. Sci. 175, 27-35 (1995).

6. P. Santoro and M. Lowenberg, "Coalescence of drops with tangentially mobile interfaces: effect of ambient flow," Ann. N.Y. Acad. Sci. 1161, 277-291 (2009).